Coral Reefs: Diversity on Display

Zebra Shark

At first glance, the distinctive and beautifully-marked Zebra Shark (Stegostoma
fasciatum) seems poorly named. The term ‘zebra’ hardly seems descriptive of
this amber to cream-colored shark with leopard-like spots.

The Zebra Shark’s equine moniker refers only to juveniles of the species,
which — up to a length of about 20 inches (50 centimetres) — are dark brown
or purple with white or pale yellow vertical bars and spots. By the time
this species reaches a length of about 35 inches (90 centimetres), the dark
saddles break up into small brown spots on a yellowish background. As this
species grows, its pattern of dark spots becomes less linear and more
uniformly distributed over the body, eventually forming an attractive
pattern of leopard-like spots. The name ‘Zebra Shark’ is retained, however,
to avoid confusion with other species known colloquially as the “leopard
shark”, including the Tiger Shark (Galeocerdo cuvier)

Why does the Zebra Shark change its stripes for leopard spots? Such
radical changes in pigmentation pattern between juvenile and adult are
nearly universal among carpet sharks of the order Orectolobiformes and coral
reef fishes in general. In reef-dwelling teleosts, it is believed that
juveniles of many species wear a distinct ‘uniform’ that advertises their
low social status, indicating that they are not a threat to — and thereby
avoid being attacked by — territorial adults. But there is no conclusive
evidence of territoriality in any shark species.

The functional significance of the bold stripes of juvenile Zebra Sharks
is hindered by the fact that, although they are occasionally encountered and
a few have been collected as museum specimens, no one seems to know the
habitat where the juveniles typically live. One source suggests they may
live in water deeper than 165 feet (50 metres), while another attests that —
at least off India — they are very common in shallow, coastal waters while
adults of this species are typically found farther offshore. At present, the
evidence seems to favor the latter contention. If this is true, then
juvenile Zebra Sharks may form loose aggregations as a protective measure,
relying on their strongly-marked bodies writhing and wriggling near one
another to visually confuse a would-be predator. Perhaps observations of
juvenile Zebra Sharks in the wild or more detailed capture records will
eventually shed some light on this mystery.

Adolescent and adult Zebra Sharks are most often encountered during
daylight hours, lying quietly on sandy bottoms between coral heads. This
species often lies in passes or channels through the reef crest, places
where tidal flow squeezes and accelerates oxygen-bearing seawater. In these
passes, Zebra Sharks are typically observed facing open-mouthed into the
current, propped up on their pectoral fins. By elevating their mouth and
gills above the substrate, where water flow is not impeded by friction with
the bottom, these sharks may facilitate breathing while at rest. Zebra
Sharks are probably nocturnal, actively hunting for prey at night.

In India, this species is known as the “monkey-mouth shark”. This
wonderfully evocative name refers to the ‘face’ formed on the underside of
its head, which features a blunt snout with prominent nares (possibly
suggestive of eyes?) and nasal barbels (cheek ‘whiskers’?) immediately in
front of the short, transverse mouth. The small mouth and thick throat and
gill muscles of this shark enable it to forcefully suck up prey animals,
including snails, bivalves, crabs, shrimps, and small teleosts. These are
grasped between multiple rows of trident-shaped teeth which are stout enough
to crush even the sturdiest of tough-shelled prey.

The long, flexible body and tail of the Zebra Shark enable it to enter
reef crevices and caves to capture concealed prey. Nearly half the total
length of this shark consists of its resplendent, pennant-like caudal fin.
As in many other orectoloboid sharks, the elongate caudal fin of the Zebra
Shark is almost parallel with the body axis and has virtually no lower lobe.
Such a tail has a low thrust angle and is relatively inefficient for
high-speed or long-distance swimming. But, coordinated with the other fins,
it is wonderful for fine control of bodily yaw, pitch, and roll. Zebra
Sharks typically swim with an exaggerated, eel-like wriggle. In areas with a
gentle but persistent through-current, it is not uncommon to see one of
these sharks ‘surfing’, delicately adjusting its fins to remain more-or-less
stationary in the water column with its tail waving sinuously, like a silk
ribbon caught in a draft.

Each summer and autumn in the tropical and warm temperate waters of
Australia, Zebra Sharks migrate into shallow coastal areas. There, they lie
on sandy bottoms or sand patches between coral reefs, forming loose
aggregations. These aggregations may be associated with Zebra Shark
breeding. Zebra Shark mating behavior has never been observed, but must be
incredibly sinuous and graceful to behold. Courtship in this species
probably involves the successful male grasping a receptive female’s pectoral
fin in his mouth. Copulation probably occurs on the bottom, the male pushing
the female onto her back, then arching his body to bring his belly along
hers before inserting a single clasper into her vent. But all this is mere
speculation on my part.

Shortly after mating, female Zebra Sharks lay four or more egg cases.
These egg cases are dark brown or purplish-black, measuring 7 inches (17
centimetres) long by 3 inches (8 centimetres) wide and about 2 inches (5
centimetres) thick. In addition, each egg case has along both sides thick
tufts of fine, hair-like fibers that probably serve to anchor it to the
substrate. After an undetermined gestation period, those egg cases lucky
enough to avoid being eaten will hatch. One end of each egg case splits
open. Then a tiny, zebra-striped shark wriggles forth, ready to find food
and evade predators. If it is successful at meeting these twin challenges,
the young Zebra Shark will grow, eventually exchanging its birthday suit for
the leopard spots that adorned its parents.